During the sewage and aqueous industrial waste treatment process, it is often environmentally advantageous to treat sludges in large tanks known as digesters. Sludge is held in these air tight tanks for typically two to three weeks, while they are broken down by anaerobic bacteria. The result of this break down is a reduction in sludge volume, together with the liberation of large volumes of methane gas.
Digester gas is completely saturated with water, and although of variable quality usually has constituents of about 50% methane, 48% carbon dioxide, 2% nitrogen and significant trace quantities of hydrogen sulphide and mercaptans.
In order to promote digestion at optimum efficiency, it is necessary to keep the digester at an elevated temperature, and to keep it thoroughly mixed. Using a compressor to draw off gas from the top of the digester, and to re-inject it back at the base is a very efficient way of doing this – it also has the advantage that the heat of compression is passed into the sludge, helping to keep it up to temperature. Other advantages of gas mixing over mechanical mixing are a lower initial capital cost, simpler installation, higher gas production, and a reduced tendency to be fouled by rags and detritus. Rotary vane type compressors are best technology for this application, although other types can be used in certain circumstances.